Polylactic acid (PLA) is a synthetic aliphatic polyester derived from renewal resources, such a corn, sugar beet and cassava, which can ultimately be degraded under composting conditions.
Although attempts have been made to utilize PLA for various end-use applications, PLA is known to be brittle and exhibit low toughness, which can result in low impact strength products or articles. Impact resistance of PLA can be modified by using existing polymeric impact modifiers; however, currently available polymeric impact modifiers always decrease transparency of PLA material. For example a liquid plasticizer can be used at high content (>15%) to improve impact resistance of PLA, however during the life time of the PLA blend, there is migration of the plasticizer. Furthermore, it is no possible to use liquid plasticizer in crystalline PLA without avoiding re-crystallization that boosts plasticizer migration.
Impact modifiers such as rubber, poly(ethylene glycol) (PEG), acrylonitrile-butadiene-styrene copolymer (ABS) have been tested. Nevertheless, the immiscibility between these impact modifying additives and the PLA matrix is a major drawback.
Commercially available BioStrength® 150 a methyl methacrylate-butadiene-styrene co-polymer (MBS) is one of the best currently available impact modifiers for PLA; however haze of the resulting PLA material increases from 5, for pure PLA to 95 when 15% w/w of BioStrength® 150 is added. Another commercial product. BioStrength® 280, an acrylic core shell impact modifier, is a less efficient impact modifier, although the resulting PLA material is said to remain transparent. Nevertheless, the present inventors observed that addition 0115% w/w of BioStrength®280, produces a material with a haze of 44.
Plasticizers are additives that increase the fluidity of a material. Commonly used plasticizers, are tributyl citrate (TBC) and acetyl tributyl citrate (ATBC). However, when 15% TBC or ATBC were mixed with PLA, the present inventors observed a plasticizer migration after storage for a few days at room temperature in summer time (25-30° C.).
In turn, Grinsted plasticizer is said not to migrate, however the present inventors observed whitening of PLA-containing Grinsted plasticizer during storage. Additionally differential scanning calorimetry (DSC) showed beginning of crystallization on aged material, Therefore, it can be said that this material is not stable during longer period of time.
Other commonly used polymer modifiers are styrene block copolymers, such as poly(styrene-butadiene-styrene), or SBS, Further studies performed by the present inventors, showed that a blend of PLA with SBS exhibited a total incompatibility even at a concentration as low as 10% w/w of SBS.
There is therefore a need to improve the compositions of the prior art.